The artwork is in the public domain. The photo of it is not. You can take your own photo, but if you want to use a photo someone else took, then you need their permission.
A publisher can certainly impose their own restrictions before publishing something to lower liability, but that does not mean the photographs themselves are not public domain, as GP comment explicitly showed.
If you have any case law since then that proves otherwise, great, but an anecdote is not proof enough.
1. Hire a photographer to take a photo of the artwork that you want to use. You have to pay the photographer, and you still need the museum's permission to access the work in a setting where you can take a good photo suitable for publication. You probably can't just snap a photo while touring the museum and use that.
2. Use a photo that the museum has and pay them and/or agree to their terms. Maybe you could take that photo and then share it because it's technically public domain, but guess what's going to happen the next time you want to use one of their photos then?
Certainly, they are under no obligation to take the photo, deliver the photo, host the photo on a server, but once you have the photo, there is no legal mechanism from preventing you from using it. Your arguments in this thread have gone: copyright protects photos of public domain works -> well people can still sue you -> well people don't have to give you access. You have arrived at the truth: museums don't have to take photos or share them with you. They own the physical artwork and can control physical access to it. I don't think that was in dispute.
That’s why it’s actually a big deal and a Good Thing that museums are making these images available online. It’s not as simple as “they were public domain anyway”.